Natural gas liquefaction process

ABSTRACT

The invention relates to a natural gas liquefaction process and particularly to one suited to use offshore. The invention provides a natural gas liquefaction apparatus wherein a carbon dioxide based pre-cooling circuit is provided in a cascade arrangement with a main cooling circuit. The invention also extends to a natural gas liquefaction apparatus wherein a main cooling circuit uses as a refrigerant a gas stream, at least a portion of which is derived from a raw natural gas source.

[0001] The present invention relates to a natural gas liquefactionprocess and particularly, but not exclusively, to one suited for useoffshore.

[0002] Natural gas can be obtained from the earth to form a natural gasfeed which must be processed before it can be used commercially.Normally the gas is first pretreated to remove or reduce the content ofimpurities such as carbon dioxide, water, hydrogen sulphide, mercury,etc.

[0003] The gas is often liquefied before being transported to its pointof use to provide liquefied natural gas (LNG). This enables the volumeof gas to be reduced by about 600 fold, which greatly reduces thetransportation costs. Since natural gas is a mixture of gases, itliquefies over a range of temperatures. At atmospheric pressure, theusual temperature range within which liquefaction occurs is between−165° C. and −155° C. Since the critical temperature of natural gas isabout −80° C. to −90° C., the gas cannot be liquefied purely bycompressing it. It is therefore necessary to use cooling processes.

[0004] It is known to cool natural gas by using heat exchangers in whicha gaseous refrigerant is used. One well-known method comprises a numberof cooling cycles, typically three, in the form of a cascade. In suchcascades, refrigeration may be provided by methane, ethylene and propanein sequence. Another type of cascade arrangement which uses mixedrefrigerant streams is described in WO 98/48227. Another known systemuses a mixture of hydrocarbon gases, such as propane, ethane and methanein a single cycle and a separate propane refrigeration cycle to providecooling of the mixed refrigerant and natural gas.

[0005] It will be appreciated that the use of hydrocarbons asrefrigerants poses a safety issue and this is particularly significantin the offshore environment, where it is highly undesirable to havelarge liquid hydrocarbon inventories in what is inevitably a confinedspace.

[0006] As an alternative, Thomas et al (U.S. Pat. No. 6,023,942)discloses a natural gas liquefaction process in which carbon-dioxide maybe used as a refrigerant. However, this process is not suitable forlarge scale or offshore applications since it relies not on a cascadearrangement but on an open-loop expansion process as the primary meansof cooling the LNG stream. Expansion processes such as this do not allowsufficiently low temperatures to be attained, and hence the LNG has tobe kept at very high pressures to maintain it in liquid form. Both froma safety and an economic point of view, these high pressures are notsuitable for industrial production of LNG, and particularly not forlarge scale or offshore applications.

[0007] A further alternative would be a nitrogen cycle based process,but this has the significant disadvantage that the thermal efficiency ismuch lower than a hydrocarbon based system. In addition, becausenitrogen has a low heat transfer co-efficient, a large heat transferarea is required to dissipate the waste heat from the process into acooling medium. Consequently, despite the safety hazards involved,hydrocarbon-based refrigeration cycles continue to be used.

[0008] According to the present invention there is provided a naturalgas liquefaction apparatus, wherein a carbon dioxide based pre-coolingcircuit is provided in a cascade arrangement with a main coolingcircuit.

[0009] By means of this arrangement, it is possible to use saferrefrigerants in the main cooling circuit, compared to theabove-mentioned hydrocarbon based cycles, whilst reducing the energyconsumption involved by using such cycles.

[0010] As discussed above, in a cascade arrangement, cooling is carriedout by a series of refrigeration cycles which are typically in the formof a closed loop system. Typically, the arrangement is such that thenatural gas stream passes through a series of interrelated heatexchangers which are arranged such that at least one coolant streampasses through a plurality of heat exchangers in sequence. Preferablytwo or more refrigeration streams are used and the arrangement may thenbe such that one stream passes through one heat exchanger and a furtherstream passes through that heat exchanger and a further one. Where threeheat exchangers are provided there may be three coolant streams with onepassing through each heat exchanger, one through two of these, etc.

[0011] Furthermore, it is possible to derive the carbon-dioxide from thenatural gas feed. As mentioned above, carbon-dioxide is normally removedfrom the gas during the pre-treatment stage and is usually vented to theatmosphere or reinjected back to nearby reservoirs. Thus, not only isthe CO₂ readily available, but also the environmentally undesirablerelease of CO₂ may to some extent reduced.

[0012] The CO₂-based pre-cooling circuit may contain other gases, forexample hydrocarbons, but preferably these amount to less than 5 mol %,and it is particularly preferred for the gas to be essentially pure CO₂.

[0013] Furthermore, the use of CO₂ means that it is possible usecomparatively high suction pressures for the refrigerant mediumcompressors (of the order of 6 to 10 bara), such that small diameterpiping can be used which results in a more compact design. Together,these features lead to a very small footprint for the cryogenic sectionof the plant (i.e. that part operating at below −40° C.), which is ofparticular importance in an offshore application.

[0014] Preferably, the suction of the refrigeration compressors receivesunheated, cold refrigerant medium directly from the cryogenic heatexchangers.

[0015] Preferably, the main cooling circuit comprises a nitrogen richbased circuit, i.e. one which uses a refrigerant which is rich innitrogen. This may be essentially pure nitrogen such that therefrigerant gas which is flowing through the expansion loops of the maincooling circuit forms a non-combustible mixture. The nitrogen gas may beobtained from the atmosphere.

[0016] Thus, in a preferred embodiment, the main cooling cycle(s)comprise nitrogen rich based expansion loop(s). In these loops therefrigerant is a nitrogen rich composition and the refrigerant is itselfcooled using an expansion loop mechanism.

[0017] In order to improve the efficiency of operation of the apparatus,other gases, such as hydrocarbons may be mixed with the nitrogen. Themain cooling circuit preferably contains a plurality of cycles and thefirst of these may preferably be richer in nitrogen than subsequentcycles. This is because the first cycle is the coldest cycle, andadvantageously contains more nitrogen than the subsequent warmer cycles.The nitrogen rich stream may be a mixture of nitrogen with any othersuitable gas, preferably hydrocarbons such as C₁ to C₅ hydrocarbons,particularly methane, ethane, propane, butane, pentane, ethylene orpropylene. For example, the first cycle may use essentially purenitrogen, or as little as 30 mol % nitrogen. Generally the refrigerantstream may comprise about 50-100 mol % nitrogen and about 0-50 mol %hydrocarbons, but preferably at least 80 mol % nitrogen is used whichmay be combined with methane and ethane (for example 80 mol % nitrogen,15 mol % methane, 5 mol % ethane). The subsequent cycles may containsignificantly less nitrogen and correspondingly more hydrocarbon gas,for example, as little as 5 to 20 mol % nitrogen may be used insubsequent cycles.

[0018] A further advantage of these embodiments of the invention is thatthe required hydrocarbon make-up is easily available from the LNGproduction process, without the need for a dedicated fractionationsystem as is usually required in the prior art. Thus, although flammablehydrocarbon gases are used as refrigerants in these embodiments, largeinventories of them need not be specially stored. Rather, they may beobtained from the natural gas itself.

[0019] In addition, nitrogen and/or hydrocarbon used in the system as arefrigerant can also be obtained from the natural gas. The use of such asupply in this context is believed to be inventive, and so viewed from adifferent aspect, the invention provides a natural gas liquefactionapparatus wherein a cooling circuit uses as a refrigerant a gas streamat least a portion of which is derived from the raw natural gas source.For example, nitrogen or hydrocarbon or a nitrogen enriched refrigerantstream may be obtained from the same raw natural gas source as thenatural gas to be liquefied. It is preferred that a nitrogen enrichednatural gas stream is used. It is also preferred that the gas stream hasa portion made up from the light hydrocarbon stream from the reflux drumof a heavy hydrocarbon removal tower.

[0020] In general, the raw natural gas stream will contain a sufficientamount of hydrocarbons to satisfy the requirements of the refrigerantcooling stream. However, since generally more nitrogen is required inthe refrigerant stream, it may be necessary to supplement the nitrogenfrom the raw natural gas with nitrogen from other sources. Nitrogen gasis readily available and may for example be obtained from the cryogenicseparation of air. It will be appreciated that a suitable mixture ofnitrogen and hydrocarbon obtained from the raw natural gas source, andif necessary topped up by additional nitrogen gas, may be used as aready and reliable source of the refrigerant stream. In such a case, theapparatus is considerably simplified.

[0021] Hydrocarbons can be recycled from various sources in the gasliquefaction process. For example, the makeup hydrocarbon may be takenfrom the reflux drum of the heavy hydrocarbon-removal tower. Preferablythe make-up hydrocarbon for the gas stream is taken partly from theoverhead hydrocarbon removal tower and partly from the reflux drum ofthe heavy hydrocarbon removal tower, the heavier hydrocarbons being moresuitable for the later cooling stages. This forms a highly efficientdual flow carbon dioxide pre-cooled mixed refrigeration process.

[0022] In a preferred embodiment of the invention, the firstnitrogen-based cycle includes hydrocarbons derived from the overhead ofthe hydrocarbon removal tower. The later cycles may comprisehydrocarbons that have been refluxed. In both cases it has been foundthat a useful refrigerant gas mainly free of aromatic hydrocarbons isproduced. It will be appreciated that the presence of aromatics isundesirable because of their tendency to freeze. The bottom product fromthe heavy hydrocarbon removal unit can be routed to the condensatestabiliser column.

[0023] As a refinement of the invention, the bottoms from thehydrocarbon removal units may be sent to a condensate stabilising unit.

[0024] Typically, the above described apparatus is arranged to providethree separate streams, namely condensate, LNG and LPG, in line withconventional practice. However, it has now surprisingly been found thatonly two separate product streams need to be produced: LNG and acombined condensate/LPG stream (unstabilised condensate product). Suchproducts have the considerable advantage that they can be transportedmore easily than the three conventional product streams. Thus, it may besimpler and more cost effective to transport an unstabilised condensateproduct stream than to transport the LPG and stabilised condensatecomponents separately. This is itself regarded as inventive, and soviewed from another aspect, therefore, the invention provides a methodof producing liquefied natural gas (LNG) wherein an unstabilisedcondensate product stream is produced. From a still further aspect, theinvention provides a method of transporting natural gas product,comprising the provision of an unstabilised condensate product stream,and the subsequent transportation of said stream, for example by pipe,ship, tanker, etc.

[0025] As mentioned above, the use of refrigerants (in particularnitrogen and hydrocarbons) obtained from the gas feed is regarded asproviding further inventive matter and therefore, viewed from a furtheraspect, the invention provides a method of liquefying natural gaswherein gas(es) obtained from the natural gas feed are used asrefrigerants. In preferred forms the refrigerants thereby obtainedinclude carbon dioxide, nitrogen and/or hydrocarbons as discussed abovewhich may be used in cascading cycles.

[0026] A further and general advantage of the invention is that theprocessing steps are not sensitive to the motions that occur in anyfloating LNG plant and the process is simple to operate in all transientoperation situations.

[0027] Embodiments of the events will now be described, by way ofexample only, and with reference to the accompanying drawings, in which:

[0028]FIG. 1 schematically represents the natural gas liquefactionprocess in accordance with a first embodiment of the invention.

[0029]FIG. 2 schematically represents an alternative natural gasliquefaction process in accordance with a second embodiment.

[0030]FIG. 3 is a flowsheet of the LNG plant as a whole incorporatingthe LNG liquefaction system as shown in FIG. 1.

[0031]FIG. 4 is a flowsheet of the LNG plant as a whole incorporatingthe LNG liquefaction system as shown in FIG. 2.

[0032]FIG. 5 is a flowsheet of the LNG plant as a whole producing onlytwo product streams: LNG and unstabilised condensate product.

[0033] The natural gas liquefaction process shown in FIG. 1 is designedfor use off-shore and comprises essentially a natural gas circuit withpre-cooling, a liquefaction circuit and a sub-cooling refrigerationcircuit.

[0034] The pre-treated natural gas stream N1 is pre-cooled down to 8-30°C. in the water cooler CW1 at 30-70 barg. The pre-cooled natural gas N2is introduced into cryogenic heat exchangers E1A, E1B and E1C where itis partially condensed and pre-cooled down to about −30 to 50° C. Afterthis pre-cooling step, the natural gas N8 is liquefied in the cryogenicheat exchanger E2 at about −80° C. to −100° C. Then the liquefiednatural gas N10 is sub-cooled to about −150° C. to −160° C. in thecryogenic heat exchanger E3. After the sub-cooling, the LNG steam N11 isexpanded close to the atmospheric pressure in the Joule Thompson valveN12 (or alternatively in a cryogenic liquid turbine). The LNG is furtherrouted to a nitrogen removal unit before it is pumped to an LNG storage.

[0035] The pre-cooling refrigerant is dry carbon dioxide which ispreferably taken from a CO₂ removal part of the pre-treatment process,but it could be taken from other sources e.g. CO2 can be imported. TheCO₂-stream provides cooling for the natural gas N2, liquefactionrefrigerant L2 and sub-cooling refrigerant S2 down to a level of about−30 to −55° C. In order to achieve these temperatures, vaporisation ofthe carbon dioxide within the cooling circuit must take place. Thecritical temperature of carbon dioxide therefore imposes an upper limiton the temperature of the carbon dioxide streams P4, P7 and P10 whichare used in heat exchangers N3, N5 and N7. The refrigeration is providedby the compressed pre-cooling refrigerant P1 which is first condensed inthe cooler CW2 by the use of sea water. Sea water is conveniently usedbecause it is available even in remote locations in warm climates. Inpractice the cooling water in unit CW2 should be at least below about28° C. to achieve sufficient pre-cooling with carbon dioxide. Ifnecessary, seawater from the depths of the ocean may be used as thiswill be cooler than seawater at the surface. The condensed pre-coolingrefrigerant stream P3 from the drum D1 is flashed through Joule Thompsonvalves V1A, V1B and V1C in three pressure levels in cryogenic heatexchanges E1A, E1B and E1C. The vaporised pre-cooling refrigerants P5,P8 and P11 are returned through the suction drums D2, D3 and D4 to thecompressor C1 where the pre-cooling refrigerant is recompressed up to 45to 60 barg because of the three different pressure levels (5.5 to 7barg, 10 to 20 barg and 25 to 35 barg) at which pre-cooling refrigerantsP4, P7 and P10 evaporate, the streams are returned to the compressor C1at three different pressure levels. The compressor C1 is designed toreceive the low pressure stream P12 (5.5 to 7 bara) at the suction andother medium pressure streams P9 and P6 (10 to 20 bara and 25 to 35bara) at interstage positions. This improves the efficiency of thepre-cooling cycle. The required liquid hold-up for the pre-coolingcircuit is provided by the drum D1.

[0036] The liquefaction refrigerant L1 is a dry nitrogen rich streamcontaining essentially N2 (50 to 100 mol %) and light hydrocarbons (0 to50 mol %) which liquefies the natural gas at −80° C. and providescooling for sub-cooling refrigerant down to a level of −80° C. to −100°C. The refrigeration is provided by the compressed and pre-cooledliquefaction refrigerant L5 by expanding it in the expander EXP1 tolower pressure (2 to 12 bara) and low temperature (−80° C. to −130° C.)in the cryogenic heat exchanger E2. The liquefaction refrigerant L7 isheated up to about −40 to −60° C. and routed to the suction of therefrigeration compressor C2 where it is recompressed up to 30 to 50barg. The recompressed refrigerant stream L8 is cooled in the cooler CW4and compressed further in the booster compressor EXC1 from 40 to 70barg. The booster compressor EXC1 is directly coupled with the expanderEXP1. The high pressure nitrogen L1 is routed through the after coolerCW3 and the cryogenic heat exchangers E1A, E1B and E1B being cooled downabout −30 to −55° C. before it is recycled to the suction of theexpander EXP1.

[0037] The sub-cooling refrigerant cycle is designed to sub-cool the LNGso that not more than the required quantity of flash gas is producedafter expansion of the LNG in the downstream nitrogen removal unit. Thesub-cooling refrigerant is dry nitrogen rich stream containingessentially N2 (50 to 100 mol %) and light hydrocarbons (0 to 50 mol %),The refrigeration is provided by the compressed and pre-cooledsub-cooling refrigerant S6 by expanding it in the expander EXP2 to lowerpressure (2 to 12 bara) and lower temperature (−160 to −175° C.) in thecryogenic heat exchanger E3. The sub-cooling refrigerant S8 is heated upto about −80 to −100° C. and routed to the suction of the refrigerationcompressor C3 where is recompressed up to 50-60 barg. The compressor C3could be integrated with the refrigerating compressor C2 in order toreduce capital costs. The recompressed refrigerant S9 is cooled in thecooler CW6 and compressed further in the booster compressor EXC2 to60-90 barg. The booster compressor EXC2 is directly coupled with theexpander EXP2. The high pressure nitrogen rich S1 is routed through theafter cooler CW5 and the cryogenic heat exchangers E1A, E1B, E1C and E2being cooled down to about −80° C. to −100° C. before it is recycledback to the expander.

[0038] The high pressure liquefaction refrigerant L2 and sub-coolingrefrigerant S1 could be combined to a common high pressure refrigerantstream in the heat exchangers E1A, E1B and E1C if this is seen to be amore cost effective concept.

[0039] The second embodiment shown in FIG. 2 comprises essentially: anatural gas circuit with pre-cooling unit and main cooling circuits.

[0040] The pre-treated natural gas stream N1 is pre-cooled down to 8-30°C. in the water cooler CW2 at 30 to 70 barg. The pre-cooled natural gasN2 is introduced into the cryogenic heat exchangers E1A, E1B and E1Cwhere it is partially condensed and pre-cooled down to about −30 to −55°C. After the pre-cooling step, the natural gas N8 is liquefied andsub-cooled in the cryogenic heat exchanger E2 down to about −150° C. to−160° C. After the sub-cooling, the LNG stream N9 is expanded close tothe atmospheric pressure in the Joule Thompson valve N10 (oralternatively in a cryogenic liquid turbine). The LNG is further routedto a nitrogen removal unit before it is pumped to an LNG storage.

[0041] The pre-cooling refrigerant is a dry carbon dioxide taken from aCO₂ removal part of the pre-treatment process. The CO₂ stream providescooling for the natural gas N2 and the main refrigerant M2 down to alevel of about −30 to −55° C. The refrigeration is provided by thecompressed pre-cooling refrigerant P1 which is first condensed in thecooler CW1 by the sea water. The condensed pre-cooling refrigerantstream P3 from the drum D1 is flashed through Joule Thompson valves V1A,V1B and V1C in three pressure levels in cryogenic heat exchangers E1A,E1B and E1C. The vaporised pre-cooling refrigerants P5, P8 and P11 arereturned through the suction drums D2, D3 and D4 to the compressor C1where the pre-cooling refrigerant is recompressed up to 45 to 60 bargbecause of the three different pressure levels (5.5 to 7 barg, 10 to 20barg and 25 to 35 barg) at which pre-cooling refrigerants P4, P7 and P10evaporate the streams are returned to the compressor C1 at threedifferent pressure levels. The compressor C1 is designed to receive thelow pressure stream P12 (5.5 to 7 bara) at the suction and other mediumpressure streams P9 and P6 (10 to 20 bara and 25 to 35 bara) atinterstage positions. This improves the efficiency of the pre-coolingcycle. The required liquid hold-up for the pre-cooling circuit isprovided by the drum D1.

[0042] The main cooling refrigerant cycle ensures the liquefaction andsub-cooling of the pre-cooled natural gas stream N8 and auto-cooling ofthe main refrigerant itself. The main cooling refrigerant is taken fromthe overhead of the hydrocarbon removal tower and enriched with nitrogenhaving essentially the following composition: 0 to 15 mol % nitrogen, 10to 90 mol % methane, 0 to 90 mol % ethane, 0 to 30 mol % propane and 0to 10 mol % butanes.

[0043] The main cooling refrigerant MS is partially condensed in thecryogenic heat exchangers E1A, E1B and E1C and is separated to a liquidand vapour phase in the separate D5 at −30 to −55° C. The vapour phaseis the light main cooling refrigerant M8, high in nitrogen and methanecontent while the liquid phase is the heavy main cooling refrigerant M7,high in ethane and propane content. The MB is condensed and sub-cooledin the tube side of the E2 and expanded in the Joule Thompson valve V2(or in the liquid turbine) to a low pressure 0.2 to 6 barg and routed tothe shell side of the E2. The evaporation of the M11 ensures thesub-cooling of natural gas stream N9 and its own sub-cooling.

[0044] The heavy main cooling refrigerant M7 from the separator D5 issub-cooled in the tube side of the cryogenic heat exchanger E2 andexpanded through Joule Thompson valve V3 to a low pressure 0.2 to 6 bargand routed to the shell side of E2. This stream is mixed with the lightmain cooling refrigerant and the evaporation of this stream provides therefrigeration required for liquefaction of the natural gas stream andthe light main cooling refrigerant.

[0045] The evaporated and slightly superheated main cooling refrigerantM14 is routed to the suction drum D6 of the compressor C2, where it iscompressed to 6 to 20 barg, intercooled in the water cooler CW3 andfurther compressed in the C3 to 20 barg. The compressed main coolingrefrigerant M1 is desuperheated in the water cooler CW4 and re-routed topre-cooling heat exchangers E1A, E1B and E1C.

[0046] Further details of the condensation and evaporation mechanism ofthe refrigerants and LNG will be understood by a person skilled in theart having reference to the disclosure of WO 98/48227.

[0047] The overall flow scheme of the LNG plant shown in FIG. 3essentially shows the pre-treatment of the raw natural gas stream beforeit enters the LNG liquefaction system previously described in FIG. 1 toproduce the desired LNG product.

[0048] The raw natural gas feed 1 is pre-treated by processing itthrough a slug catcher 2 to remove heavy residues. Typically, the rawnatural gas may comprise 0-5 mol % nitrogen, 0-20 mol % carbon dioxide,50-100 mol % C₁, 0-10 mol % C₂, 0-10 mol % C₃, 0-10 mol % C₄ and 0-5 mol% C₅+. The heavy residues are fed to a separator 3 which produces an LPGproduct stream 4 and a stabilised condensate product stream 5. Thenatural gas stream 6 leaving the top of the slug catcher 2 is subjectedto a series of pre-treatment steps including carbon dioxide removal 7,water removal 8 and mercury removal 9, before entering the system ofheat exchangers 10 according to FIG. 1.

[0049] After passing through the heat exchanger N3, the natural gas 11passes through a heavy hydrocarbon removal unit 12 in which the lighterhydrocarbons 13 leave the top of the column 12 and pass through the heatexchanger N5 where condensation takes place. The bottoms 14 from theheavy hydrocarbons removal unit are fed into the heavy residue stream 15from the slug catcher and subsequently leave the system in the LPGproduct and stabilised condensate product streams 4 and 5. The naturalgas stream 16 after condensing in heat exchanger N5 is passed throughthe reflux drum 17 of the heavy hydrocarbon removal unit 12. The stream18 from the top of the reflux drum 17 continues through heat exchangerN7 and is topped up by some of the bottoms 19 from the reflux drum 17.The remainder of the bottoms 19 from the reflux drum 17 are recycledback into the heavy hydrocarbon removal unit 12. The heat exchanger N7provides further cooling of the liquefied natural gas stream 20. Furthercooling steps may take place in further heat exchangers (not shown) asdescribed earlier with reference to FIG. 1.

[0050] Since the refrigerant stream in the main cooling circuit of FIG.1 contains predominantly nitrogen, recycle of hydrocarbons from thenatural gas stream is not necessary and is not shown. However, ifdesired, some light hydrocarbons from 13 the top of the heavyhydrocarbon removal unit 12 or, more preferably, from the top of thereflux drum 17 could be used in a refrigerant make-up stream (notshown).

[0051]FIG. 4 shows a flow scheme of the overall LNG plant incorporatingthe liquefaction system 22 using a mixed hydrocarbon and nitrogenrefrigerant stream as shown in FIG. 2. Pre-treatment of the raw naturalgas stream 6 and the fate of the LPG product and stabilised condensateproduct streams 4 and 5 are shown in the same way as described above inrelation to FIG. 3.

[0052] However, the liquefaction system shown in FIG. 4 also contains arefrigerant make-up stream 23, 24 comprising hydrocarbons enriched withnitrogen, in accordance with the system of FIG. 2. Therefore, arefrigerant make-up stream 23 comprising hydrocarbons from the refluxdrum 17 is shown. The light hydrocarbons 13 in the stream from the topof the heavy hydrocarbon removal unit 12 passes through heat exchangerN5 and then into the reflux drum 17. From the top of the reflux drum 17,some of the natural gas stream is removed to form the refrigerantmake-up 24. Some of the heavy hydrocarbons 25 from the bottom of thereflux drum 17 are also used in the refrigerant make-up stream 23, andthe remainder is refluxed back into the heavy hydrocarbon removal unit12.

[0053] Although heat exchangers N3, N5 and N7 only are shown in thisdrawing, further heat exchangers as described in FIG. 2 may be necessaryor desired to produce the LNG product stream.

[0054]FIG. 5 shows an overall flow scheme of the LNG plant in which theraw natural gas stream is pre-treated as described in FIG. 3. Thenatural gas liquefaction system 27 in accordance with FIG. 2 is shown,and includes the refrigerant make-up streams 23,24 taken from thehydrocarbon streams from the reflux drum 17. However, the liquefactionsystem 27 shown in FIG. 1 and described above could be used instead.

[0055] The bottoms 14 from the heavy hydrocarbon removal column are fedinto the stream 15 exiting the bottom of the slug catcher, and thecombined stream 28 is fed into a condensate removal column 29. The tops30 from the condensate removal column 29 are recycled back into thenatural gas stream 6 prior to pre-treatment by carbon dioxide, water andmercury removal, 7, 8 and 9, as shown. It will be noted that a singleproduct stream is removed from the bottom of the separator in the formof an unstabilised condensate product stream 31. This product streamneed not undergo any further separation before it is transported. On thecontrary, by this means only two separate streams need be transported,compared to three in the conventional arrangement.

1. A natural gas liquefaction apparatus wherein a carbon dioxide basedpre-cooling circuit is provided in a cascade arrangement with a maincooling circuit.
 2. An apparatus as claimed in claim 1 comprising aplurality of main cooling cycles.
 3. An apparatus according to claim 1or 2, wherein the main cooling cycle(s) comprises nitrogen rich basedexpansion loop(s).
 4. A natural gas liquefaction apparatus wherein amain cooling circuit uses as a refrigerant a gas stream, at least aportion of which is derived from the raw natural gas source.
 5. Anapparatus according to claim 4 wherein a nitrogen-enriched natural gasstream is used.
 6. An apparatus according to claim 4 or claim 5 whereinsaid gas stream has a portion made-up from the light hydrocarbon streamfrom the reflux drum of a heavy hydrocarbon removal tower.
 7. Anapparatus according to any of claims 1 to 6, wherein a cycle of the maincooling circuit uses a nitrogen enriched natural gas stream where themake-up of that gas is taken partly from the overhead of a hydrocarbonremoval tower and partly from the reflux drum of the heavy hydrocarbonremoval tower.
 8. An apparatus according to any preceding claim, whereinthe suction of the refrigeration compressors receive unheated, coldrefrigerant medium directly from the cryogenic heat exchangers.
 9. Anapparatus according to any preceding claim, wherein the bottoms from thehydrocarbon removal unit are sent to a condensate stabiliser column orthe like.
 10. An apparatus as claimed in any preceding claim wherein arefrigerant stream used in the main cooling cycle comprisesapproximately 50 to 100 mol % nitrogen and about 0 to 50 mol %hydrocarbons.
 11. An apparatus as claimed in any preceding claim whereina refrigerant stream used in the main cooling cycle comprising about 0to 15 mol % nitrogen and 50 to 100 mol % hydrocarbons.
 12. A natural gasliquefaction process wherein the gas is cooled by a carbon dioxide basedpre-cooling circuit in a cascade arrangement with a main coolingcircuit.
 13. A process as claimed in claim 12, comprising the use of aplurality of main cooling cycles.
 14. A natural gas liquefaction processas claimed in claim 12 or 13, wherein the main cooling cycle uses anitrogen rich refrigerant.
 15. A process as claimed in any of claims 12to 14, wherein the main cooling circuit comprises a cycle using anitrogen enriched natural gas where the make-up is taken partly from theoverhead of a hydrocarbon removal tower and partly from the reflux drumof the heavy hydrocarbon removal tower.
 16. A process as claimed in anyof claims 12 to 15, wherein the suction of the refrigeration compressorsreceive unheated, cold refrigerant medium directly from the cryogenicheat exchangers.
 17. A process as claimed in any of claims 12 to 16,wherein the bottoms from the hydrocarbon removal unit are sent to a unitfor stabilising condensate.
 18. A method of producing liquefied naturalgas (LNG) wherein an unstabilised condensate product stream is produced.19. A method of transporting natural gas product, comprising theprovision of an unstabilised condensate product stream, and thesubsequent transportation of said stream.